Molecular Structure of the Uranyl Mineral Uranopilite - A Raman Spectroscopic Study

Raman spectroscopy of the mineral uranopilite at 298 and 77 K has been obtained and used to elucidate the structure of the mineral at the molecular level. A single intense band at 1010 cm-1 is assigned to the ν1 (SO4)2- symmetric stretching mode. Three low intensity bands in the 298 K Raman spectrum are observed at 1143, 1117 and 1097 cm-1and in the 77 K spectrum four bands at 1148, 1130, 1118 and 1106 cm-1. These bands are attributed to the ν3 antisymmetric stretching modes. A series of infrared bands are found at 1559, 1540, 1526 and 1511 cm-1 attributed to δ UOH bending modes. Three bands are observed at 843, 835 and 819 cm-1 in both 298 and 77 K spectra attributed to the ν1 symmetric stretching modes of the (UO2)2+ units. In the infrared spectra bands are observed at 941, 929 and 910 cm-1 and are assigned to the ν3 antisymmetric stretching modes of the UO2 units. These values are used to calculate UO bond distances. Multiple OH stretching modes prove a complex arrangement of OH groupings i.e. water molecules and OH- ions, and hydrogen bonding in the crystal structure of uranopilite.